Along with interconnection of large-sized power systems and various new devices, the scale and complexity of the power system are increased while power generation and power transmission are more economic and efficient; in addition, since massive distributed power generation systems are connected to a power grid, the traditional fixed power flow transmitted to a power distribution network from a power transmission network is reversed; the continuous increase of user loads needs a power flow control means to improve the existing power transmission capacity; and the power exchange between the flourish smart power grid and the power market needs frequent power flow control.
A unified power flow controller (UPFC) is the power flow control device having the best universality so far, the simplest UPFC consists of two exactly same voltage source converters connected by a common direct end, and can be regarded as a constitution of a static synchronous compensator (STATCOM) and a static synchronous series-connection compensator (SSSC), by only changing a control rule, different control functions of parallel compensation, serial compensation, phase shift, etc., can be fast realized in the meantime or respectively, and the performances of the power systems are improved.
At present, more research on the UPFC is a topological structure and control strategy directed to a mono-circuit power transmission line UPFC, and the research on the UPFC for the double-circuit transmission line is less. Since the power transmission networks in China are mostly double-circuit or multi-circuit line, the power flow optimization problem and N−1 overcurrent problem occur in the multi-circuit line, and if only one circuit of the multi-circuit line is provided with the UPFC to control the power flow of the line, a control effect is poor. Therefore, if an optimal addressing location of the UPFC is a double-circuit line, the UPFC needs to be mounted on both double circuits. If a set of whole UPFC is mounted on each circuit of the double-circuit line, each UPFC consists of a static synchronous compensator (comprising a parallel-connection transformer and a parallel-connection side converter) and a static synchronous series-connection compensator (comprising a series-connection transformer and a series-connection side converter) which are connected, and four converters and four transformers are required in total, the occupied space is larger and the investment cost is higher; and when any converter is failed, the power flow control over the double-circuit power transmission line is not realized.
In order to solve the deficiency of the UPFC applied to the double-circuit line, save investment cost and a mounting space, improve the efficiency and reliability of the UPFC, and fully play the advantage of the UPFC, there is a need of a more economic and more flexible structure.